1. Field of the Disclosure
The present disclosure relates to a semiconductor optical modulator that is operated at an ultrafast speed in a long-wavelength range and an optical modulating apparatus in which the semiconductor optical modulator is used.
2. Discussion of the Background Art
A method for combining a laser diode light source and external modulator to generate an optical signal is usually adopted in a transmitter used in a high-density multiple-wavelength optical communication system. The typical external modulator used in this kind of purpose is a LiNbO3 (hereinafter abbreviated as “LN”) modulator formed by an LN waveguide. The modulation of a refractive index by an electro-optic effect is a basic operation of the LN modulator and an optical intensity modulator in which a Mach-Zehnder interferometer is incorporated is also available in addition to a simple optical phase modulator.
Recently attention is focused on a semiconductor optical modulator that is superior to the LN modulator in miniaturization. Examples of the semiconductor optical modulator include a GaAs optical modulator in which a Schottky electrode is disposed in semi-insulating GaAs to utilize the Schottky electrode as a photoelectron waveguide and an InP/InGaAsP optical modulator in which a voltage is effectively applied to a core portion of the waveguide while light is confined using a pin heterojunction.
Although the semiconductor optical modulator has the advantage of miniaturization, unfortunately the semiconductor optical modulator has a high driving voltage, therefore, as a structure to avoid the problem, there has been proposed such an npin-type semiconductor optical modulator that both InP clad layers are formed into an n-type and a thin p-type semiconductor layer (p-type barrier layer) is inserted as a barrier layer to suppress an electron current (for example, see Patent Document 1). Because the p-type clad layer which causes an optical loss is not used in the npin-type, a relatively long waveguide can be used, and the npin-type is superior in reducing the driving voltage. Because the npin-type has a degree of freedom of optimally-designing arbitrarily a thickness of a depletion layer, advantageously matching of electric impedance and matching of electric speed/light speed are easily satisfied at the same time, and the npin-type is superior in speed enhancement.
In the npin-type semiconductor optical modulator structure, it is well known that a conductivity of an upper n-type clad layer is larger than that of conventional pin-type p-type clad layer. This means that, in the npin-type having a uniform structure, an modulation electric signal that should be applied to the waveguide only in a light travelling direction along an anode electrode above the waveguide leaks through the n-type clad layer to the waveguide in which the anode electrode does not exist. Occasionally a constant DC bias is also applied to the waveguide, and similarly the DC bias is also applied to the waveguide in which the anode electrode does not exist. Because the leakage of the electric signal or DC bias has an influence on a modulation operation, in the npin-type semiconductor optical modulator structure, a waveguide electric separation technique is adopted, where the waveguide is separated into a portion in which the electric signal and the DC bias are applied to the waveguide and a portion in which the electric signal and the DC bias are not applied to the waveguide.
FIG. 1 illustrates an example of the waveguide electric separation technique. In the npin-type semiconductor optical modulator structure of FIG. 1, a layer configuration of the waveguide 24 is formed in the order of n-p-i-n from above, and the upper n-type second clad layer 27 and p-type barrier layer 26 are partially recessed to form electric separation grooves (groove) 29-1, thereby adopting the waveguide electric separation technique.
However, in the waveguide electric separation technique adopted in the npin-type semiconductor optical modulator structure of FIG. 1, light scattering is generated by the local unevenness of the waveguide, while the electric separation is completely performed, therefore, it is necessary to solve the problem of the optical diffraction loss.
FIG. 2 illustrates an example of the waveguide electric separation technique in which the optical diffraction loss is solved. In the npin-type semiconductor optical modulator structure of FIG. 2, a layer configuration of the waveguide 24 is formed in the order of n-i-p-n from above, and a local p-type region 29-2 reaching a lower surface of an n-type first clad layer 23 is re-formed by ion implantation of a p-type dopant or regrowth method (for example, see Patent Document 2).
The nipn-type semiconductor optical modulator structure of FIG. 2 is opposite to the npin-type semiconductor optical modulator structure of FIG. 1 in the layer configuration from above of the waveguide 24. The reason will be described below. FIG. 3 illustrates an npin-type semiconductor optical modulator structure in which the waveguide electric separation technique of FIG. 2 is adopted. In the npin-type, because the local p-type region 29-3 is in contact with the p-type barrier layer 26, the waveguide electric separation cannot be performed certainly.
Specifically, the waveguide electric separation can be performed in the state in which a large DC bias is applied in the opposite direction to deplete the p-type barrier layer 26. However, in the state in which the p-type barrier layer 26 has the large total acceptor amount while the opposite DC bias is lower than a predetermined value, holes remaining in part of the p-type barrier layer 26 is neutralized, and the waveguide electric separation cannot be separated for a high-frequency electric signal. That is, even if the n-type second clad layer 27 in which the anode electrode 28 exists thereon and the n-type second clad layer 27 in which the anode electrode 28 does not exist thereon are electrically separated, the high-frequency electric signal leaks through the p-type barrier layer 26 to the waveguide 24 in the portion in which the anode electrode 28 does not exist thereon.
Because a potential at the p-type barrier layer 26 in an electrically “floating state” is unstable, a dark current changes depending on a temperature or a bias voltage, and an electric field of the waveguide 24 also changes. This has a large influence on long-term reliability of the element. Therefore the waveguide electric separation technique in which the p-type region is formed in the n-type clad layer, because the waveguide electric separation can be performed even if part of the barrier layer is neutralized, it is necessary to vertically invert the structure of the nipn-type.
Patent Document 1: Japanese Patent Application Laid-Open No. 2005-099387
Patent Document 2: Japanese Patent Application Laid-Open No. 2005-116644
However, there is a production problem in the nipn-type semiconductor optical modulator structure. Specifically, during epitaxial growth of each nipn-type semiconductor layer, an impurity background concentration of an i layer that constitutes the waveguide on the barrier layer hardly becomes a desired concentration (for example, 2×1015/cm3) or less due to the influence of the p-type dopant remaining in a growth atmosphere. Therefore, unfortunately the semiconductor optical modulator having good linearity to the electric signal input is hardly produced in the nipn-type semiconductor optical modulator structure.